1. Field of the Invention
The present invention relates to variable reluctance actuators, particularly those of the type utilized to apply a controlled force between two points for active vibration control. Furthermore, the invention relates to control systems by which such variable reluctance actuators can be operated for attenuation and damping of vibration and structure borne noise.
2. Related Art
While variable reluctance force actuators normally produce force in only one direction (i.e., that acting to reduce the air gap between the armature and the electromagnet) and are very non linear, a number of patents disclose the use of two variable reluctance actuators in conjunction with each other to produce a bidirectional actuator. Likewise, it is known to utilize force varying characteristics of tapered pole pieces together with a variable area overlap between the armature and pole piece of the electromagnet to obtain a linear behavior. Examples of such bidirectional linear actuators can be found in U.S. Pat. Nos. 3,312,842 and 4,282,501.
Another example of a variable reluctance linear actuator can be found in U.S. Pat. No. 4,656,400, wherein a Hall effect sensor signal representative of flux density in the magnetic circuit of the actuator is utilized to control the moving element of the actuator. However, the actuator of this patent is not bidirectional, requiring an opposing return force mechanism, such as a spring or fluid pressure mechanism. Such actuators possess inherent deficiencies and lack the versatility to be utilized in a wide range of active sound and vibration attenuation applications.
Of course, the use of oppositely acting pairs of actuators in conjunction with appropriate controls to obtain vibration control is also known (see, for example, U.S. Pat. Nos. 4,686,404 and 4,697,128). These arrangments are in the nature of radial magnetic bearing type devices for rotary shafts. However, no bidirectional variable reluctance actuator with an appropriate control is known which is utilizable for active attenuation of vibration and structure borne noise and which is capable of cancelling up to six degrees of freedom of structure motion through a wide range of different types of paths, not merely those associated with radially directed deviations of a rotary shaft.